# Can we create invisible things?

We are able to see this beautiful world because of light. When light gets reflected from the object, the reflected light enters our eye retina and thus we are able see that particular object from which light was reflected from. In our daily life, we see certain things which are transparent, they appear transparent because most of the light is refracted instead of getting reflected. So, there will be only very less reflected rays which enters our eye and thus enables see up to certain extent. Suppose we have mirror or any other matter which is perfectly transparent, i.e it reflects no light incident on it. If this happens, we will not be able to see that particular object! So, if this is a possibility we can create invisible things like invisible robot, etc from those perfectly reflecting matter (like ideal transparent glass).

Is this really possible? If yes what kind of things would be better to create in invisible form, like one may show their idea by displaying their model (ex. spy robot). Every one can give their idea, so that there will be value of saying why this site is for scientists!

• Can't resist claiming that I create invisible pink unicorns all the time. Prove me wrong! :-) . On the serious side, a perfectly absorbing medium (think stealth aircraft in the radar band) is invisible because it emits/reflects no light. You can see it by contrast with illuminated objects behind it, but you can't actually see it. Commented Jan 25, 2014 at 1:44
• @CarlWitthoft, We have an adjective to describe a surface that emits/reflects no light. It's "black". Using radar to search for a stealth aircraft in the sky is like using a flashlight to search for a black ball in the blackness of outer space. Not an easy task, but imagine that same black ball sitting in the grass in your back yard on a sunny day. Can you see it now? Commented Dec 15, 2015 at 14:03
• @jameslarge Well, I was a bit glib: a perfectly absorbing surface, unless it has a massive cooling heatsink behind it, will emit per black-body law. There's a rather large difference between "perfectly transparent" and "perfectly nonabsorbing": you cannot see either object but in the latter case you can infer its existence by the lack of visibility of whatever's behind it. Commented Dec 15, 2015 at 14:40

A transparent object, like glass or some plastics, is not invisible for 2 reasons:

• The index of refraction is not the same as air. In fact, no solid material has an index close enough to that of air (I found some nano-materials having 1.1 compared to air's 1.0003). Thus, the refraction will distort the image and the object will be visible because of this distortion.

• Even very transparent glass has some opacity. If you have a thin slice, it may seem transparent, but when you have a thick piece, it will absorb a significant portion of the light.

The last decade there has been much activity about so-called negative index materials which would have some of the properties you mention. A negative index material has both electric (permitivity) and magnetic susceptibility equal to -$\epsilon_0$ and -$\mu_0$, where $\epsilon_0=\mu_0$ are the dielectric constant in vacuum (so the speed of light in the material equals its vacuum value). The problem is how to manufacture such materials. They could then also be used to make "perfect" lenses. The idea was first put forward by the Russian scientist Veselago (hence the name Veselago lens) and taken up again by the British physicist John Pendry. For details, also about other systems, see http://en.wikipedia.org/wiki/Perfect_lens.

However it has turned out that such properties can only be true (if at all) for a set of discrete frequencies, so no cloacking devices for the military. Also the medium may not be absorptive in the frequency range of interest, absorption directly spoils it.

Besides the metamaterials (which can be invisible only in a certain specific frequency band, as Urgie already pointed out) truly invisible objects are made of matter that does not interact electro-magnetically.

In spirit of fffred's answer, only two things come to my mind:

* A neutron or neutrino beam (it is ideally invisible, regardless on density or diameter)
* The famous dark matter (ok, we cant create it)

• Except any tiny particle (like neutrino) is invisible in the sense that it doesn't reflect/refract enough light for us to detect. Commented Jan 25, 2014 at 1:42

Creating something perfectly invisible obviously means that it reflects no light and every light ray passes through it. If it would absorb all the light, then it could be easily distinguished from its background. So there are kind of 3 possibilities.

1.) You create something that blends and camouflages in the background. Well this is not invisible but invisible to the human eye.

2.) You create a smart object that detects its surroundings and if a light ray touches it, it absorbs it and then emits it from the other side following the path of the light. This would be king of impossible because there are so many light rays that it is nearly impossible to detect every single one of them.

3.) As already mentioned above, under certain conditions its easy to make something invisible. But to make something invisible in every surrounding would require artificial intelligence as well.

That may depend on what you call "invisible". How about a system of direction-sensitive Lytro cameras all around a body and projectors transmitting whatever the cameras on the other side of the body sees in the direction opposite to the sensed one? As of today it's a mildly challenging but doable engineering project. Would such a system count?